Know Your Friends

Peristenus digoneutis,
Parasite of Tarnished Plant Bug

Peristenus digoneutis is a braconid wasp
parasite of tarnished plant
bugs (Lygus spp.). In its native
Europe it parasitizes L. rugulipennis, but in North
America it parasitizes L. lineolaris.

Beginning about 1978,
entomologists at the USDA-ARS European Biological Control
Laboratory, now located in Montpellier, France, collected Peristenus
wasps in Europe and shipped them to the USDA-ARS Beneficial
Insects Research Lab in Newark, Delaware. Reared wasps were first
released in northern New Jersey in 1979, with continued
introductions through 1987, for control of tarnished plant bug on
alfalfa. By 1983 Peristenus was recovered in low
numbers and was determined to be established there by 1988.
Within a few years parasitism levels began to rise, reaching 50%
by 1990-92. By 1988-90 it had become the dominant parasite
species in northern New Jersey and was spreading to the
northeast. Since then this parasite has been found in six new
states (Pennsylvania, Massachusetts, New Hampshire, Vermont,
Connecticut and New York).

P. digoneutis is a 2-3 mm
long brown wasp. The adults emerge from overwintering cocoons in
May-June. The adults live 2-4 weeks. The female wasp lays eggs
singly in the abdomen of lygus bug nymphs. The eggs hatch in 5-7
days. After the wasp larva completes its development in about
7-10 days, the mature larva emerges from the dying host to spin a
cocoon just under the soil surface. The second generation of
adults emerge in July to attack the next generation of lygus bug.
There may also be a partial third generation in August, with each
generation synchronized with the host's major generations. In
each generation an increasing proportion of individuals enter
diapause (delay emergence from their cocoons until the following
spring) to carry the species through possible periods of host
scarcity and the winter.

This reduction in plant bug
numbers in alfalfa benefits not only that crop, but may
indirectly benefit valuable fruit and vegetable crops by reducing
the number of tarnished plant bug adults which fly into and
damage them, especially when nearby alfalfa is harvested.
Researchers in New York and New Hampshire are working with the
USDA Lab in Newark to determine if P. digoneutis will
fly into strawberries and parasitize bugs there or if alfalfa must be nearby.
If P. digoneutis is found to disperse to and remain in
perennial fruit crops such as peaches, strawberries, apples, and raspberries, or
annual crops such as lima and dry beans and several seed crops, even greater biological control
of tarnished plant bug should be realized.

P. digoneutis persists
well in alfalfa fields, despite 3-4 harvests per year. Weeds
around fields may also be a suitable habitat for the adult
parasites, especially if they harbor tarnished plant bug. Flowers
such as the composites Erigeron spp. and goldenrod,
crucifers, and umbelliferous flowers such as Queen Anne's lace
provide nectar as food for the wasps. These plants may be
important in the conservation of this wasp so it can become an
effective biological control agent in other crops.

It is very difficult to rear P.
digoneutis, so commercial availability of this parasitoid is
unlikely. However, it is dispersing naturally, so most suitable
areas will eventually have self-sustaining, natural populations
of this parasitoid. It may eventually move into the Midwest on
its own, or be could possibly be established from releases of
wasps collected in the East.

- Bill Day, USDA-ARS
Beneficial Insects Research Lab, Newark, DE and Susan Mahr,
University of Wisconsin